direx

<?php

$numbers = [
    '64 654 руб.',
    '231 654 руб.',
    '9 879 руб.',
    '164 руб.',
    '2 815 руб.'
];

var_dump(
    getMinNumberFromArray($numbers, true), // float(164)
    getMinNumberFromArray($numbers) // string(11) "164 руб.",
);

/**
 * Поиск минимального числа в массиве
 * @param array $numbers
 * @param boolean $format
 * @return string|flaot|null
 */
function getMinNumberFromArray(array $numbers, $format = false)
{
    $minIndex = null;
    $minNum = null;

    foreach ($numbers as $k => $number) {
        $number = (float)preg_replace("/[^0-9,.-]/", "", $number);

        if (is_null($minNum) || $number < $minNum) {
            $minIndex = $k;
            $minNum = $number;
        }
    }

    if(is_null($minNum)){
        return null;
    }

    return ($format) ? $minNum : $numbers[$minIndex];
}

function getNameFromNumber($num)
    {
        $numeric = $num % 26;
        $letter = chr(65 + $numeric);
        $num2 = intval($num / 26);
        if ($num2 > 0) {
            return getNameFromNumber($num2 - 1) . $letter;
        } else {
            return $letter;
        }
    }